1. Field of the Invention
The invention relates to the calibration of coordinate positioning machines, such as machine tools or coordinate measuring machines.
Such machines typically comprise an arm (e.g. a toolholder) movable in three dimensions relative to a table or base on which, for example, a workpiece is supported. It is frequently desired to calibrate such machines to determine the accuracy to which for example a toolholder on a machine tool may describe a circular trajectory with respect to a fixed centre on the workholder of the machine.
2. Description of Related Art
A known device for performing such a calibration operation is described in U.S. Pat. No. 4,435,905. The device comprises an elongate telescopic bar provided with a ball at each end. In use, each of the balls is retained in a socket provided on the toolholder and the workholder respectively, and the toolholder is then driven in a circular path about the centre of the ball retained in the socket on the workholder. A single axis transducer provided on the bar measures any variation in the centre-to-centre spacing of the balls, and thus determines the extent to which the tool holder path varies from a circular path. It is also known from GB 2,210,978 to provide a ball on each of the tool holder and workholder, and an interconnecting transducer between the two balls. The interconnector comprises a bar having a socket at one end for universally pivotally engaging one of the balls, and a plurality of forks at the other end of the bar for engaging the other of the balls to enable both universal pivotal motion, and linear motion of the other of the balls relative to the bar. A pair of single axis transducers are provided on the bar to determine the variation in spacing between the two balls as the tool holder is driven in a circular path.
In each of the above mentioned calibration devices, it is a requirement in order to accurately calibrate the machine to determine the centre of motion about which, for example, the tool holder is to be driven. In these known devices, this centre is defined by the centre of the ball which is secured to the workholder of the machine. Existing methods for determining the position of the centre of the ball retained on the workholder are based on trial and error. For example, with the toolholder and workholder engaged with the bar, the toolholder is moved to diametrically opposing points about a position which is assumed to be the centre of the ball retained on the workholder. The difference in the outputs between the single axis transducer readings at each of the two locations of the toolholder is used to determine a distance by which the centre of the ball retained on the workholder is to be moved in order to position it coincidentally with the position at which its centre is assumed to be. Such methods are time consuming.